KVM: x86: hyper-v: Make Hyper-V emulation enablement conditional

Hyper-V emulation is enabled in KVM unconditionally. This is bad at least
from security standpoint as it is an extra attack surface. Ideally, there
should be a per-VM capability explicitly enabled by VMM but currently it
is not the case and we can't mandate one without breaking backwards
compatibility. We can, however, check guest visible CPUIDs and only enable
Hyper-V emulation when "Hv#1" interface was exposed in
HYPERV_CPUID_INTERFACE.

Note, VMMs are free to act in any sequence they like, e.g. they can try
to set MSRs first and CPUIDs later so we still need to allow the host
to read/write Hyper-V specific MSRs unconditionally.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210126134816.1880136-14-vkuznets@redhat.com>
[Add selftest vcpu_set_hv_cpuid API to avoid breaking xen_vmcall_test. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

5 files changed, 29 insertions, 7 deletions

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index f47bbd4b0837..c8f2592ccc99 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -179,6 +179,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	vcpu->arch.cr4_guest_rsvd_bits =
 	    __cr4_reserved_bits(guest_cpuid_has, vcpu);
 
+	kvm_hv_set_cpuid(vcpu);
+
 	/* Invoke the vendor callback only after the above state is updated. */
 	static_call(kvm_x86_vcpu_after_set_cpuid)(vcpu);
 
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 333bd200f186..11d16fe52324 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -37,6 +37,9 @@
 #include "trace.h"
 #include "irq.h"
 
+/* "Hv#1" signature */
+#define HYPERV_CPUID_SIGNATURE_EAX 0x31237648
+
 #define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
 
 static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
@@ -1473,6 +1476,9 @@ int kvm_hv_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data, bool host)
 {
 	struct kvm_hv *hv = to_kvm_hv(vcpu->kvm);
 
+	if (!host && !vcpu->arch.hyperv_enabled)
+		return 1;
+
 	if (kvm_hv_msr_partition_wide(msr)) {
 		int r;
 
@@ -1488,6 +1494,9 @@ int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host)
 {
 	struct kvm_hv *hv = to_kvm_hv(vcpu->kvm);
 
+	if (!host && !vcpu->arch.hyperv_enabled)
+		return 1;
+
 	if (kvm_hv_msr_partition_wide(msr)) {
 		int r;
 
@@ -1701,9 +1710,20 @@ ret_success:
 	return HV_STATUS_SUCCESS;
 }
 
-bool kvm_hv_hypercall_enabled(struct kvm *kvm)
+void kvm_hv_set_cpuid(struct kvm_vcpu *vcpu)
+{
+	struct kvm_cpuid_entry2 *entry;
+
+	entry = kvm_find_cpuid_entry(vcpu, HYPERV_CPUID_INTERFACE, 0);
+	if (entry && entry->eax == HYPERV_CPUID_SIGNATURE_EAX)
+		vcpu->arch.hyperv_enabled = true;
+	else
+		vcpu->arch.hyperv_enabled = false;
+}
+
+bool kvm_hv_hypercall_enabled(struct kvm_vcpu *vcpu)
 {
-	return to_kvm_hv(kvm)->hv_guest_os_id != 0;
+	return vcpu->arch.hyperv_enabled && to_kvm_hv(vcpu->kvm)->hv_guest_os_id;
 }
 
 static void kvm_hv_hypercall_set_result(struct kvm_vcpu *vcpu, u64 result)
@@ -2036,8 +2056,7 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
 			break;
 
 		case HYPERV_CPUID_INTERFACE:
-			memcpy(signature, "Hv#1\0\0\0\0\0\0\0\0", 12);
-			ent->eax = signature[0];
+			ent->eax = HYPERV_CPUID_SIGNATURE_EAX;
 			break;
 
 		case HYPERV_CPUID_VERSION:
diff --git a/arch/x86/kvm/hyperv.h b/arch/x86/kvm/hyperv.h
index 4bc2ab0caa92..f28e7fb88765 100644
--- a/arch/x86/kvm/hyperv.h
+++ b/arch/x86/kvm/hyperv.h
@@ -92,7 +92,7 @@ static inline u32 kvm_hv_get_vpindex(struct kvm_vcpu *vcpu)
 int kvm_hv_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data, bool host);
 int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host);
 
-bool kvm_hv_hypercall_enabled(struct kvm *kvm);
+bool kvm_hv_hypercall_enabled(struct kvm_vcpu *vcpu);
 int kvm_hv_hypercall(struct kvm_vcpu *vcpu);
 
 void kvm_hv_irq_routing_update(struct kvm *kvm);
@@ -141,6 +141,7 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm,
 
 void kvm_hv_init_vm(struct kvm *kvm);
 void kvm_hv_destroy_vm(struct kvm *kvm);
+void kvm_hv_set_cpuid(struct kvm_vcpu *vcpu);
 int kvm_vm_ioctl_hv_eventfd(struct kvm *kvm, struct kvm_hyperv_eventfd *args);
 int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
 		     struct kvm_cpuid_entry2 __user *entries);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a75d01523c4f..1e5c304d14ab 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8162,7 +8162,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
 	if (kvm_xen_hypercall_enabled(vcpu->kvm))
 		return kvm_xen_hypercall(vcpu);
 
-	if (kvm_hv_hypercall_enabled(vcpu->kvm))
+	if (kvm_hv_hypercall_enabled(vcpu))
 		return kvm_hv_hypercall(vcpu);
 
 	nr = kvm_rax_read(vcpu);
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index b414fed1576b..b08a688e56f9 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -369,7 +369,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 
 	/* Hyper-V hypercalls get bit 31 set in EAX */
 	if ((input & 0x80000000) &&
-	    kvm_hv_hypercall_enabled(vcpu->kvm))
+	    kvm_hv_hypercall_enabled(vcpu))
 		return kvm_hv_hypercall(vcpu);
 
 	longmode = is_64_bit_mode(vcpu);